- Results are presented of studies on the electric fields in and around ion-exchange membranes. In a charge membrane, through which ions are transported, several electric field may be superimposed such as local fields around polyelectrolyte chains or near the walls of charged pores, Donnan potentials between membrane and solution phases, double layers on the phase boundaries, and externally applied fields. Fields, concentration, and bulk velocity distributions are mutually dependent. Different simplifying assumptions have allowed model calculations which give at least a good qualitative description of real systems. Conclusions from studies on the influence of inhomogenous membrane surfaces, substantial deviations from equilibrium distributions and instabilities at high current densities are summarized.